In a GaN-type FET, and in particular, a high frequency GaN-type FET, and especially, a GaN-type FET fit for millimeter wave band or for submillimeter wave band, in order to achieve a high frequency performance, structure of an HJFET with use of two-dimensional electron gas is used widely. Specifically, construction that an AlGaN/GaN heterojunction is provided under a gate electrode and electrons supplied from the AlGaN which functions as an electron supply layer are stored at this heterojunction interface to form two-dimensional electron gas is generally adopted. In such a case, as substrate 1, a sapphire substrate, a SiC substrate, a Si substrate, and the like are used, an extremely thin nucleation layer, which functions as a growth nucleus of epitaxial growth, is formed on its surface, and a GaN-type epitaxial layer is grown on this nucleation layer. Conventionally, an AlN nucleation layer is selected as a nucleation layer, a GaN layer is subsequently formed as a buffer layer, and also a GaN layer is continuously grown up as a channel layer. By growing up AlGaN electron supply layer 5 on a surface of GaN layer 4 that is continuously formed as the buffer layer and channel layer, band gap difference ΔEc in a conduction band therebetween is use to achieve confinement of two-dimensional electron gas at a heterojunction interface between the GaN layer of the channel layer and AlGaN electron supply layer 5. FIG. 4 illustrates the construction of a multilayered epitaxial film for preparation of an AlGaN/GaN HJFET in which a GaN layer is adopted as this buffer layer and a GaN layer of the channel layer is continuously formed by epitaxial growth, and, FIG. 5 illustrates an example of device structure of the AlGaN/GaN HJFET prepared using a multilayered epitaxial film with said construction.